In modulators of the kind given above, comprising a wafer of optoelectronic material with lightwave conductors, it is known to form the electrodes as three rectangular plates, extending along the lightwave conductors. The electrodes are connected to an electric microwave source which genrates an electric field through the conductors, causing their refractive index to be changed. This also changes the length of the optical path in the lightwave conductors and this is used to modulate the lightwave obtained when the lightwaves in the two lightwave conductors are superimposed. The lightwave has a greater velocity than the microwave which limits the modulators' frequency range, which is the so-called "walk-off effect". The upper boundary frequency can be raised by making the electrodes shorter, although this requires a higher microwave voltage which is difficult to generate. In U.S. Pat. No. 4,448,479 there is shown a modulator with relatively complicated electrodes, which has a higher upper boundary frequency for a moderate modulation voltage. This modulator has the disadvantage, apart from the complicated electrodes, that its modulating ability is poor for individual electric pulses, i.e. its pulse response is poor. This has great disadvantages in information transmission, for example, in which binary numbers are transferred by discrete pulses. The modulator in the patent also has limited modulating ability for low frequencies of the modulating microwave.